Meaning
Mechanical response of a thermoplastic to varying speeds of deformation indicates its capacity to distribute stress during shaping. This material parameter, known as strain rate sensitivity, describes how the yield stress of a polymer increases as the rate of deformation rises. It is a fundamental property that dictates the stability of the stretching process in both extrusion and blow moulding.
Deformation Control
When a polymer displays positive strain rate sensitivity, any localized thinning will draw faster than the surrounding areas, causing the local stress to rise. This localized increase in stress halts the localized deformation and forces the surrounding thicker regions to stretch instead. This self-stabilizing mechanism prevents the formation of thin spots and ensures a uniform wall thickness across the entire bottle body.
It is particularly important during high-speed, multi-axial stretching.
Processing Benefit
Resins with a higher sensitivity index are easier to process over a wider range of temperatures. They tolerate slight fluctuations in preform heating without producing defective containers. This makes the moulding process more forgiving and reduces the overall rate of scrap.
Testing Requirement
Measuring this sensitivity involves running tensile tests on the polymer at multiple pulling speeds. These experiments must be performed under controlled temperature conditions that replicate the actual blow moulding window. The resulting data is then used to calibrate the constitutive equations used in simulation software.
